Final answer:
The specified allowance in cam-operated valves designed to prevent cam jams is called over-travel allowance, which ensures reliability and tolerance over time.
Step-by-step explanation:
The extra roller-plunger travel designed into cam-operated valves to prevent cam jam is called over-travel allowance. This feature is critical in valve design and ensures that the cam and plunger mechanism can complete its intended motion even if there is a slight misalignment or wear over time. By providing this over-travel, the design helps to prevent a condition where the cam could become stuck or jammed, leading to valve malfunction. In the context of mechanical engineering, this is an example of designing for reliability and tolerance.